FIG. 1 is a structural schematic diagram of a camera based touch screen in the prior art, comprising a touch detection region 10, two cameras 111 and 112 respectively installed on two corners of the touch detection region 10, two light sources 121 and 122 respectively installed at locations adjacent to the cameras 111 and 112, a regression reflection bar 13 installed around the touch detection region 10, and a processing module 14 connected with the cameras 111 and 112. The regression reflection bar 13 reflects light emitted by the light source 121 to the camera 111, and reflects light emitted by the light source 122 to the camera 112; the cameras 111 and 112 respectively collect image data and send the image data to the processing module 14 which processes the image data to determine location information of a touch object. The touch screen may be used to obtain location information of a plurality of touch objects.
FIG. 2 is a schematic diagram of brightness of the image data collected by the camera 111 in the touch screen as shown in FIG. 1 when there is no touch object in the touch detection region. When there is no touch object in the touch detection region 10, there does not exist any darker area in the image data collected by the camera 111, and similarly, there also does not exist any darker area in the image data collected by the camera 112. FIG. 3 is a schematic diagram of brightness of the image data collected by the camera 111 in the touch screen as shown in FIG. 1 when there is one touch object in the touch detection region. When there is one touch object in the touch detection region 10, since the touch object obstructs light emitted by the light source 121 to the regression reflection bar 13, the regression reflection bar 13 cannot reflect light to the camera 111, so there is a dark bar at the location corresponding to the touch object in the image data collected by the camera 111, the dark bar being the image of the touch object. Similarly, there is also a dark bar at the location where the touch object is in the image data collected by the camera 112. And then, the processing module 14 determines location information of the touch object based on the image data collected by the cameras 111 and 112.
FIG. 4 is a schematic diagram of brightness of the image data collected by the camera 111 in the touch screen as shown in FIG. 1 when a touch object is near to the camera 111. When a touch object is near to the camera 111, since light reflected by the touch object itself is stronger, it results in a bright bar 41 instead of a dark bar at the location corresponding to the touch object in the image data collected by the camera 111. Due to shadows on both sides of the touch object, there are dark bars 42, 43 respectively on both sides of the location of the touch object. Here, the dark bar 42, 43 is called “crack dot”. In some cases, there will be more than three “crack dots”; at the time, the touch object is far away from the camera 112, and there will be a dark bar at the location where the touch object is in the image data collected by the camera 112. When the processing module 14 determines location information of the touch object according to the image data collected by the cameras 111 and 112, it will obtain location information of two touch objects, and in some cases, even three or more touch objects, which leads to misjudgment.